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MICROBIOLOGY LECTURE SERIESLUZ GREGORIA LAZO-VELASCO, MD

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GENERAL PROPERTIES OF

VIRUSES

Smallest infectious agents (20nm-300nmin diameter)

Contain only one kind of nucleic acid(RNA or DNA) as their genome

Nucleic acid is encased in a protein shellwhich may be surrounded by a lipd-containing membrane

VIRION entire infectious unit

Inert in the extracellular environment;replicate only in living cells

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VIRUSES

Viral nucleic acid contains informationnecessary for programming the

infected host cell to synthesize virus-specific macromolecules required forthe production of viral progeny

During the replicative cycle, numerous

copies of viral nucleic acid and coatproteins are produced

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VIRUSES

Coat proteins assemble together toform the capsid (encases and

stabilizes the viral nucleic acid againstthe extracellular environment;facilitates the attachment andpenetration by the virus upon contact

with new susceptible cells)

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VIRUSES

Rich in diversity viruses vary greatlyin structure, genome organizationand expression, strategies of

replication and transmission Host range maybe broad or extremely

limited

Known to infect unicellular organismssuch as mycoplasmas, bacteria andalgae and all higher plants andanimals

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TERMS AND DEFINITIONS IN

VIROLOGY

CAPSID protein shell, or coat, thatencloses the nucleic acid genome

CAPSOMERES morphologic unitsseen in the electron microscope onthe surface of icosahedral virusparticles

DEFECTIVE VIRUS virus particlethat is functionally deficient in someaspect of replication

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VIROLOGY

ENVELOPE lipid-containingmembrane that surrounds some virusparticles

NUCLEOCAPSID protein-nucleicacid complex representing thepackaged form of the viral genome

STRUCTURAL UNITS basic proteinbuilding blocks of the coat; usually acollection of >1 non-identical proteinsubunit

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VIROLOGY

SUBUNIT a single folded viralpolypeptide chain

VIRION complete virus particle;serves to transfer the viral nucleic acidfrom one cell to another

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EVOLUTIONARY ORIGIN OF

VIRUSES

Origin of viruses not known

2 theories:

1. Viruses may be derived fromDNA or RNA nucleic acid components ofhost cells that became able to replicateautonomously and evolve

independently

2. Viruses may be degenerateforms of intracellular parasites

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CLASSIFICATION OF VIRUSES

BASIS OF CLASSIFICATION1. Virion morphology, size, shape, type of

symmetry, presence or absence of peplomers,and presence or absence of membranes

2. Virus genome properties, including type of

nucleic acid (DNA or RNA), size of genome (kbor kbp), strandedness (single or double),whether linear or circular, sense (positive,negative, ambisense), segments (number,size), nucleotide sequence, G + C content, andpresence of special features [repetitiveelements, isomerization, 5'-terminal cap, 5'-terminal covalently linked protein, 3'-terminalpoly(A) tract]

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BASIS OF CLASSIFICATION3. Genome organization and replication,

including gene order, number and position ofof open reading frames, strategy of replication(patterns of transcription, translation), and

cellular sites (accumulation of proteins, virionassembly, virion release)

4. Virus protein properties, number, size, andfunctional activities of structural andnonstructural proteins, AA sequence,modifications (glycosylation, phosphorylation,myristylation), and special functional activities(transcriptase, reverse transcriptase,neuraminidase, fusion activities)

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BASIS OF CLASSIFICATION5. Antigenic properties

6. Physicochemical properties of the virion,including molecular mass, buoyant

density, pH stability, thermal stability, andsusceptibility to physical and chemicalagents, especially ether and detergents

7. Biologic properties, including natural hostrange, mode of transmission, vectorrelationships, pathogenicity, tissuetropisms, and pathology

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UNIVERSAL SYSTEM OF VIRUS

TAXONOMY

Viruses are separated into FAMILIES on thebasis of virion morphology, genomestructure and strategies of replication

Virus family names have the suffix viridae Genera subdivisions based on biological,

genomic, physicochemical or serologicdifferences; genus names carry the suffix virus

Subfamilies have been defined in severalfamilies

Order

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DNA VIRUSES

A. Parvoviruses

Human parvovirus B19 aplasticcrisis, fifth disease, fetal death

B. AnellovirusesC. Polyomaviruses

JC virus progressive multifocalleukoencephalopathy

BK virus nephropathy in transplantrecipients

Merkel cell virus - Merkel cell skin CA

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DNA VIRUSES

D. Papillomaviruses

wart viruses

E. Adenoviruses- acute respiratorydiseases, conjuctivitis ,gastroenteritis

F. Hepadnaviruses acute and

chronic hepatitis

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DNA VIRUSES

G. Herpesviruses

Herpes simplex type 1 & 2 (oral &genital lesions)

Varicella-zoster virus (chickenpox &shingles)

Cytomegalovirus

Epstein-Barr virus (infectious

mononucleosis, assoc. with human neoplasms)Human herpesvirus 6 & 7 (T

lymphocytic)

Human herpesvirus 8 (Kaposi sarcoma)

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DNA VIRUSES

H. Poxviruses

smallpox

vaccinia

molluscum contangiosumcowpox

monkeypox

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RNA VIRUSES

A. Picornaviruses

Enteroviruses (polioviruses,coxsackieviruses, and echoviruses,

rhinoviruses (common colds) andhepatoviruses (Hepatitis A)

B. Astroviruses- gastroenteritis

C. Caliciviruses

Noroviruses (Norwalk virus) epidemic acute gastroenteritis

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RNA VIRUSES

D. Hepeviruses

Human Hepatitis E virus

E. Picobirnaviruses

F. ReovirusesRotaviruses (wheel-shaped appearance;

gastroenteritis)

G. Arboviruses and Rodent-Borne Viruses

Dengue Virus

Yellow fever virus

West Nile fever virus

Encephalitis virus

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RNA VIRUSES

H. Togaviruses

Rubella virus

I. Flaviviruses

Hepatitis C virusJ. Arenaviruses

K. Coronaviruses

colds

SARS (severe acute respiratorysyndrome)

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RNA VIRUSES

L. Retroviruses

AIDS

M. Orthomyxoviruses

Influenza virusesN. Bunyaviruses

O. Bornaviruses

P. Rhabdoviruses

Rabies virus

Q. Paramyxoviruses

Mumps, measles, parainfluenza,metapneumo & respiratory syncytial viruses

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RNA VIRUSES

R. Filoviruses

Marburg virus severe hemorrhagic

Ebola virus fever in Africa

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PRINCIPLES OF VIRUS STRUCTURE

Cubic Symmetry All cubic symmetry observed with

animal viruses is of the icosahedralpattern, the most efficient

arrangement for subunits in a closedshell

Helical Symmetry

protein subunits are bound in a periodic

way to the viral nucleic acid, winding itinto a helix; the filamentous viralnucleic acid-protein complex(nucleocapsid) is then coiled inside alipid-containing envelope

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PRINCIPLES OF VIRUS STRUCTURE

Complex Structures Some virus particles do not exhibit

simple cubic or helical symmetrybut are more complicated in

structure.

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CHEMICAL COMPOSITION OF

VIRUSES VIRAL PROTEIN

Facilitate transfer of the viral nucleicacid from one host to another

Serve to protect the viral genome

against inactivation by nucleases Participate in the attachment of the

virus particle to a susceptible cell

Provide structural symmetry of the

virus particle Determines the antigenic

characteristics of the virus

Some viruses carry enzymes (which

are proteins) inside the virions

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VIRUSES VIRAL NUCLEIC ACID

Viruses contain a SINGLE kind ofnucleic acid either DNA or RNAthat encodes the genetic information

necessary for replication of the virus. The genome may be single-stranded

or double-stranded, circular orlinear, and segmented or

nonsegmented. The type of nucleic acid, its

strandedness, and its size are majorcharacteristics used for classifyingviruses into families

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Viral RNAs exist in several forms

may be a single linear molecule(picornaviruses)

several segments of RNA that maybe loosely associated within thevirion (orthomyxoviruses)

The isolated RNA of viruses with

positive-sense genomes (ie,picornaviruses, togaviruses) isinfectious, and the moleculefunctions as an mRNA within the

infected cell

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The isolated RNA of the negative-sense RNA viruses, such asrhabdoviruses and orthomyxoviruses,

is not infectious The sequence and composition of

nucleotides of each viral nucleic acidare distinctive. Many viral genomes

have been sequenced. The sequencescan reveal genetic relationshipsamong isolates

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Viral nucleic acid may becharacterized by its G + C content.

DNA viral genomes can be analyzed

and compared using restrictionendonucleases, enzymes that cleaveDNA at specific nucleotidesequences; each genome will yield a

characteristic pattern of DNAfragments after cleavage with aparticular enzyme

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VIRUSES

VIRAL LIPID ENVELOPES

A number of different viruses contain

lipid envelopes as part of theirstructure

The lipid is acquired when the viralnucleocapsid buds through a cellular

membrane in the course of maturation

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VIRUSES

VIRAL GLYCOPROTEINS

Viral envelopes contain glycoproteins

In contrast to the lipids in viralmembranes, which are derived fromthe host cell, the envelopeglycoproteins are virus-encoded.

the sugars added to viral glycoproteinsoften reflect the host cell in which thevirus is grown

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VIRUSES

VIRAL GLYCOPROTEINS

It is the surface glycoproteins of anenveloped virus that attach the virus

particle to a target cell by interactingwith a cellular receptor

often involved in the membrane fusionstep of infection

important viral antigens

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CULTIVATION OF VIRUSES

Many viruses can be grown in cell culturesor in fertile eggs under strictly controlledconditions

Growth of virus in animals is still used for

the primary isolation of certain viruses andfor studies of the pathogenesis of viraldiseases and of viral oncogenesis.

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The fundamental process ofviral infection is the viralreplicative cycle.

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The cellular response tothat infection may

range

No apparenteffect

Cytopathologywithaccompanyingcell death

Hyperplasiaor cancer

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Viral disease Some harmful abnormality that results from viral

infection of the host organism.

Clinical disease consists of overt signs and symptoms.

Syndrome a specific group of signs and symptoms.

Inapparent (subclinical) Viral infections that fail to produce any symptoms in

the host.

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Important Features

of Acute Viral DiseasesLocal Infections Systemic Infections

Specific disease example Respiratory (rhinovirus) Measles

Site of pathology Portal of entry Distant site

Incubation period Relatively short Relatively long

Viremia Absent PresentDuration of immunity Variablemay be short Usually lifelong

Role of secretory antibody(IgA) in resistance

Usually important Usually not important

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Most viral infections do not result inthe production of disease.

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Important principles that pertain to

viral disease include the following:

Many viral infections are subclinical.

The same disease may be produced by a variety of viruses.

The same virus may produce a variety of diseases.

The disease produced bears no relationship to viral morphology.

The outcome in any particular case is determined by both viral andhost factors and is influenced by the genetics of each.

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Viral pathogenesis

The interaction of viral and host factors that leads

to disease production. Disease pathogenesis subset of events

during an infection that results in diseasemanifestation in the host

A virus is pathogenic for a particular host if itcan infect and cause signs of disease in thathost.

A strain of a certain virus is more virulentthan another strain if it commonly producesmore severe disease in a susceptible host.

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Steps in viral pathogenesis

1. Entry into the host2. Primary viral replication

3. Viral spread4. Cellular injury5. Host immune response6. Viral clearance or establishment

of persistent infection7. Viral shedding

Common Routes

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Common Routes

of Viral Infection in Humans

Route of Entry Virus Group Produce LocalSymptoms at Portal ofEntry

Produce GeneralizedInfection Plus SpecificOrgan Disease

Respiratory tract Parvovirus B19

Adenovirus Most types

Herpesvirus Epstein-Barr virus,herpes simplex virus

Varicella virus

Poxvirus Smallpox virus

Picornavirus Rhinoviruses Some enteroviruses

Togavirus Rubella virus

Coronavirus Most types

Orthomyxovirus Influenza virus

Paramyxovirus Parainfluenza viruses,respiratory syncytialvirus

Mumps virus, measlesvirus

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Common Routes

of Viral Infection in HumansRoute of Entry Virus Group Produce Local

Symptoms at Portal ofEntry

Produce GeneralizedInfection Plus SpecificOrgan Disease

Mouth, intestinal tract Adenovirus Some types

Herpesvirus Epstein-Barr virus,herpes simplex virus

Cytomegalovirus

Picornavirus Some enteroviruses,including poliovirus andhepatitis A virus

Reovirus Rotaviruses

C

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Common Routes

of Viral Infection in HumansRoute of Entry Virus Group Produce LocalSymptoms at Portal

of Entry

Produce Generalized Infection PlusSpecific Organ Disease

Skin

Mild trauma Papillomavirus Most types

Herpesvirus Herpes simplex virus

Poxvirus Molluscumcontagiosum virus, orfvirus

Injection Hepadnavirus Hepatitis B

Herpesvirus Epstein-Barr virus, cytomegalovirus

Retrovirus Human immunodeficiency virus

Bites Togavirus Many species, including easternequine encephalitis virus

Flavivirus Many species, including yellow fevervirus

Rhabdovirus

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Entry & Primary Replication

Viruses usually replicate at the primary site ofentry.

Someproduce disease at the portal of entry

and have no necessity for further systemicspread.

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Many viruses produce disease at sites distantfrom their point of entry

The most common route is via the

bloodstream or lymphatics. Presence of virus in the blood -viremia

Virions may be free in the plasma or

associated with particular cell types The viremic phase is short in many viral

infections.

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Viruses tend to exhibit organ and cell

specificities tropism determines thepattern of systemic illness produced during a

viral infection Tissue & cell tropism by a given virus usually

reflect the presence of specific cell surface

receptors for that virus

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Destruction of virus-infected cells in the target tissuesand physiologic alterations produced in the host bythe tissue injury are partly responsible for thedevelopment of disease.

Some tissues Can rapidly regenerate, such as intestinal epithelium.

Can withstand extensive damage better than others, suchas the brain.

Some physiologic effects may result from nonlethalimpairment of specialized functions of cells, such asloss of hormone production.

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General symptoms associated with many viralinfections, such as malaise and anorexia, mayresult from host response functions such as

cytokine production. Clinical illness is an insensitive indicator of

viral infection.

Inapparent infections by viruses are verycommon.

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The host either succumbs orrecovers from viral infection.

OR

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Recovery mechanisms

Innate immune response

Adaptive immune responses

Interferon and other cytokines, humoral andcell-mediated immunity, and possibly otherhost defense factors are involved.

The relative importance of each componentdiffers with the virus and the disease.

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In acute infections, recovery is associatedwith viral clearance.

However, there are times when the host

remains persistently infected with the virus(chronic or latent).

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This is a necessary step to maintain a viralinfection in populations of hosts.

Shedding usually occurs from the body surfacesinvolved in viral entry.

Shedding occurs at different stages of diseasedepending on the particular agent involved.

It represents the time at which an infected

individual is infectious to contacts. In some viral infections, such as rabies, humans

represent dead-end infections, shedding doesnot occur.

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Nonspecific host defense mechanisms areusually elicited very soon after viral infection.

The most prominent among the innate

immune responses is the induction ofinterferons.

These responses help inhibit viral growth during

the time it takes to induce specific humoral andcell-mediated immunity.

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Both humoral and cellular components of theimmune response are involved in control of viralinfections.

Viruses elicit a tissue response different from the

response to pathogenic bacteria. Polymorphonuclear leukocytes form the principal

cellular response to the acute inflammation causedby pyogenic bacteria.

Infiltration with mononuclear cells and lymphocytescharacterizes the inflammatory reaction ofuncomplicated viral lesions.

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Virus-encoded proteins serve as targets for the immuneresponse. Virus-infected cells may be lysed by cytotoxic T lymphocytes as

a result of recognition of viral polypeptides on the cell surface.

Humoral immunity protects the host against reinfection by

the same virus. Neutralizing antibody directed against capsid proteins

blocks the initiation of viral infection, presumably at thestage of attachment, entry, or uncoating.

Secretory IgA antibody important in protecting against infection by viruses through the

respiratory or gastrointestinal tracts.

Viruses have evolved a variety of ways that serve to

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Viruses have evolved a variety of ways that serve tosuppress or evade the host immune response andthus avoid being eradicated.

Infect cells of the immune system and abrogating theirfunction (HIV).

Infect neurons that express little or no class I MHC(herpesvirus).

Encode immunomodulatory proteins that inhibit MHCfunction (adenovirus, herpesvirus).

Inhibit cytokine activity (poxvirus, measles virus). Mutate and change antigenic sites on virion proteins

(influenza virus, HIV). Downregulate the level of expression of viral cell

surface proteins (herpesvirus).

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Antiviral Chemotherapy

Interferons

Viral Vaccines

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Limitations:

Antiviral agents must be capable of selectivelyinhibiting viral functions without damaging the

host. Many rounds of virus replication occur during the

incubation period and the virus has spread beforesymptoms appear

making a drug relatively ineffective.

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Can be used to treat established infectionswhen vaccines would not be effective.

Antivirals are needed to reduce morbidity and

economic loss due to viral infections and totreat increasing numbers ofimmunosuppressed patients who are at

increased risk of infection.

Examples of Antiviral Compounds

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Used for Treatment of Viral Infections:Drug Nucleoside Analog Mechanism of Action Viral Spectrum1

Acyclovir Yes Viral polymerase inhibitor Herpes simplex, varicella-zoster

Amantadine No Blocks viral uncoating Influenza A

Cidofovir No Viral polymerase inhibitor Cytomegalovirus, herpessimplex, polyomavirus

Didanosine (ddI) Yes Reverse transcriptaseinhibitor

HIV-1, HIV-2

Foscarnet No Viral polymerase inhibitor Herpesviruses, HIV-1, HBV

Fuzeon No HIV fusion inhibitor (blocksviral entry)

HIV-1

Ganciclovir Yes Viral polymerase inhibitor Cytomegalovirus

Indinavir No HIV protease inhibitor HIV-1, HIV-2

Lamivudine (3TC) Yes Reverse transcriptaseinhibitor

HIV-1, HIV-2, HBV

Nevirapine No Reverse transcriptaseinhibitor

HIV-1


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Used for Treatment of Viral Infections:Drug Nucleoside Analog Mechanism of Action Viral Spectrum1

Ribavirin Yes Perhaps blocks capping ofviral mRNA

Respiratory syncytial virus,influenza A and B, Lassafever, hepatitis C, others

Ritonavir No HIV protease inhibitor HIV-1, HIV-2

Saquinavir No HIV protease inhibitor HIV-1, HIV-2

Stavudine (d4T) Yes Reverse transcriptaseinhibitor

HIV-1, HIV-2

Trifluridine Yes Viral polymerase inhibitor Herpes simplex,cytomegalovirus, vaccinia

Valacyclovir Yes Viral polymerase inhibitor Herpesviruses

Vidarabine Yes Viral polymerase inhibitor Herpesviruses, vaccinia,HBV

Zalcitabine (ddC) Yes Reverse transcriptaseinhibitor

HIV-1, HIV-2, HBV

Zidovudine (AZT) Yes Reverse transcriptaseinhibitor

HIV-1, HIV-2, HTLV-1

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Comprise the majority of available antiviralagents.

MOA:

They inhibit nucleic acid replication by inhibitionof polymerases for nucleic acid replication.

Some analogs can be incorporated into thenucleic acid and block further synthesis or alter its

function. Analogs can inhibit cellular enzymes as well as

virus-encoded enzymes.

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The most effective analogs are those able tospecifically inhibit virus-encoded enzymes, withminimal inhibition of analogous host cellenzymes.

Resistance: Virus variants resistant to the drug usually arise over

time, sometimes quite rapidly.

The use of combinations of antiviral drugs can delaythe emergence of resistant variants (eg, "triple drug"therapy used to treat HIV infections).

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Examples of nucleoside analogs include

acyclovir (Acycloguanosine),

lamivudine (3TC),

ribavirin,

vidarabine (Adenine Arabinoside),

and zidovudine (azidothymidine; AZT).

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Nucleotide analogs differ from nucleosideanalogs in having an attached phosphategroup.

Their ability to persist in cells for long periodsof time increases their potency.

Example:

Cidofovir (HPMPC)

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Nevirapine was the first member of the class ofnonnucleoside reverse transcriptase inhibitors.

It does not require phosphorylation for activity

and does not compete with nucleosidetriphosphates.

MOA:

Bind directly to reverse transcriptase and disruptingthe enzyme's catalytic site.

Resistant mutants emerge rapidly.

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Saquinavir

The first protease inhibitor to be approved fortreatment of HIV infection.

A peptidomimetic agent designed bycomputer modeling as a molecule that fitsinto the active site of the HIV protease

enzyme.

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MOA

Inhibit the viral proteasethat is required at the

late stage of thereplicative cycle

cleave the viralgag andgag-polpolypeptide

precursors

form the mature virioncore

activate the reversetranscriptase that will be

used in the next round ofinfection.

yields noninfectious virus

particles

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Protease inhibitors include

Indinavir

Ritonavir

etc

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Fuzeon

a large peptide

MOA:

Blocks the virus and cellular membrane fusionstep involved in entry of HIV-1 into cells.

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These synthetic amines specifically inhibit influenza A viruses by blockingviral uncoating.

They must be administered prophylactically to have a significantprotective effect.

Amantadine andRimantadine

An organic analog of inorganic pyrophosphate

Selectively inhibits viral DNA polymerases and reverse transcriptases atthe pyrophosphate-binding site.

Foscarnet(Phosphonoformic

Acid, PFA)

An inhibitor of poxviruses. It was the first antiviral agent to be described and contributed to the

campaign to eradicate smallpox.

It blocked a late stage in viral replication, resulting in the formation ofimmature, noninfectious virus particles.

Methisazone

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Host-coded proteins that are members of thelarge cytokine family and which inhibit viralreplication.

They are produced very quickly (within hours)in response to viral infection or otherinducers.

Are one of the body's first responders in thedefense against viral infection.

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Interferons are central to the innate antiviralimmune response.

Also modulate humoral and cellular

immunity.

Have broad cell growth regulatory activities.

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Three general groups

IFN- type I (viral IFN)

The IFN- family is large, being coded by at least 20

genes in the human genome IFN- type I (viral IFN)

IFN- type II (immune IFN)

** the IFN- and IFN- families are coded byone gene each.

P i f H I f

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Properties of Human Interferons

Type

Property Alpha Beta Gamma

Current nomenclature IFN- IFN- IFN-

Former designation Leukocyte Fibroblast Immune interferon

Type designation Type I Type I Type II

Number of genes thatcode for family

20 1 1

Principal cell source Most cell types Most cell types Lymphocytes

Inducing agent Viruses; dsRNA Viruses; dsRNA Mitogens

Stability at pH 2.0 Stable Stable LabileGlycosylated No Yes Yes

Introns in genes No No Yes

Homology with IFN- 8095% 30% < 10%

P i f H I f

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Properties of Human Interferons

Type

Chromosomal locationof genes

9 9 12

Size of secreted protein(number of aminoacids)

165 166 143

IFN receptor IFNAR IFNAR IFNGR

Chromosomal locationof IFN receptor genes

21 21 6

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The different interferons are similar in size,but the three classes are antigenically distinct.

IFN- and IFN- are resistant to low pH.

IFN- and IFN- are glycosylated, but thesugars are not necessary for biologic activity,so cloned interferons produced in bacteria are

biologically active.

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Dendritic cells are potent interferon producers;

under the same virus challenge conditions,

dendritic cells can secrete up to 1000x moreinterferon than fibroblasts.

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Interferons are produced by all vertebrate species. Normal cells do not generally synthesize interferon

until they are induced to do so. Infection with viruses is a potent insult leading to

induction; RNA viruses are stronger inducers of interferon than DNA

viruses. Interferons also can be induced by double-stranded RNA,

bacterial endotoxin, and small molecules such as tilorone.

IFN- not produced in response to most viruses but is induced

by mitogen stimulation.

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IFN- and IFN- synthesized by many cell types.

IFN-

produced mainly by lymphocytes, especially T cellsand natural killer (NK) cells.

Dendritic cells are potent interferon producers;

under the same virus challenge conditions, dendriticcells can secrete up to 1000x more interferon thanfibroblasts.

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Interferon does not protect the virus-infectedcell that produces it, and interferon itself isnot the antiviral agent.

Rather, interferon moves to other cells whereit induces an antiviral state by prompting thesynthesis of other proteins that actuallyinhibit viral replication. Interferon molecules

bind to specific cell surface receptors ontarget cells.

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Receptor binding triggerstyrosine phosphorylation

and activation oftranscription factors (STATproteins) in the cytoplasm

Translocate into the nucleusand mediate transcriptionof interferon-inducible

genes (which occurs withinminutes after interferon

binding).

Synthesis of severalenzymes believed to be

instrumental in thedevelopment of the

antiviral state.

Several pathways appear to be

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p y pp

involved:

A dsRNA-dependent protein kinase, PKR, whichphosphorylates and inactivates cellular initiation factoreIF-2 and thus prevents formation of the initiation complexneeded for viral protein synthesis;

An oligonucleotide synthetase, 2-5A synthetase, which

activates a cellular endonuclease, RNase L, which in turndegrades mRNA;

A phosphodiesterase, which inhibits peptide chainelongation;

Nitric oxide synthetase, which is induced by IFN- inmacrophages. These explanations, however, fail to revealwhy the antiviral state acts selectively against viral mRNAsand not cellular mRNAs.

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Other steps in viral replication may also beinhibited by interferon.

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Interferons are almost always host species-specific in function but are not specific for agiven virus.

When interferon is added to cells prior toinfection, there is marked inhibition of viralreplication but nearly normal cell function.

Interferons are extremely potent, so that very

small amounts are required for function.

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May block induction of expression of interferon (herpesvirus,papillomavirus, filovirus, hepatitis C virus, rotavirus);

May block the activation of the key PKR protein kinase(adenovirus, herpesviruses);

May activate a cellular inhibitor of PKR (influenza,poliovirus);

May block interferon-induced signal transduction

(adenovirus, herpesviruses, hepatitis B virus); May neutralize IFN- by acting as a soluble interferon receptor

(myxoma virus).

Specific viral proteins

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93/105

The purpose of viral vaccines is to utilize theimmune response of the host to prevent viraldisease.

Vaccination is the most cost-effective methodof prevention of serious viral infections.

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94/105

Immunity to viral infection is based on thedevelopment of an immune response to specificantigens located on the surface of virus particles orvirus-infected cells.

For enveloped viruses, the important antigens are thesurface glycoproteins.

Although infected animals may develop antibodiesagainst virion core proteins or nonstructural proteins

involved in viral replication, that immune response isbelieved to play little or no role in the development ofresistance to infection.

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95/105

The pathogenesis of a particular viral infectioninfluences the objectives of immunoprophylaxis.

Mucosal immunity (local IgA)

is important in resistance to infection by viruses that

replicate exclusively in mucosal membranes (rhinoviruses,influenza viruses, rotaviruses).

Viruses that have a viremic mode of spread (polio,hepatitis, measles)

are controlled by serum antibodies. Cell-mediated immunity also is involved in protection

against systemic infections (measles, herpes).

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96/105

Killed-Virus Vaccines

Attenuated Live-VirusVaccines

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97/105

Made by purifying viral preparations to acertain extent and then inactivating viralinfectivity in a way that does minimal damage

to the viral structural proteins Mild formalin treatment is frequently used.

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98/105

Advantages

There is no reversion tovirulence by the vaccine virusand that vaccines can be

made when no acceptableattenuated virus is available.

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99/105

Disadvantages

Extreme care is required in their manufacture tomake certain that no residual live virulent virus is

present in the vaccine. The immunity conferred is often brief and must beboosted, which not only involves the logisticproblem of repeatedly reaching the persons in needof immunization but also has caused concern aboutthe possible effects (hypersensitivity reactions) ofrepeated administration of foreign proteins.

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100/105

Disadvantages

Parenteral administration of killed-virusvaccine, even when it stimulates circulating

antibody (IgM, IgG) to satisfactory levels, hassometimes given limited protection becauselocal resistance (IgA) is not induced adequatelyat the natural portal of entry or primary site of

multiplication of the wild virus infectioneg,nasopharynx for respiratory viruses, alimentarytract for poliovirus

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101/105

Disadvantages

The cell-mediated response to

inactivated vaccines is generally poor. Some killed-virus vaccines have inducedhypersensitivity to subsequent infection,perhaps owing to an unbalanced immune

response to viral surface antigens thatfails to mimic infection with natural virus.

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102/105

Utilize virus mutants that antigenicallyoverlap with wild-type virus but are restrictedin some step in the pathogenesis of disease.

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103/105

Advantage

Act like the natural infection with regard totheir effect on immunity.

They multiply in the host and tend tostimulate longer-lasting antibodyproduction, to induce a good cell-mediated

response, and to induce antibodyproduction and resistance at the portal ofentry.

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104/105

Disadvantages The risk of reversion to greater virulence during multiplication

within the vaccinee. Although reversion has not proved to be aproblem in practice, its potential exists.

Unrecognized adventitious agents latently infecting the culturesubstrate (eggs, primary cell cultures) may enter the vaccinestocks. Viruses found in vaccines have included avian leukosisvirus, simian polyomavirus SV40, and simian cytomegalovirus.The problem of adventitious contaminants may be circumventedthrough the use of normal cells serially propagated in culture

(eg, human diploid cell lines) as substrates for cultivation ofvaccine viruses.

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105/105

Disadvantages

The storage and limited shelf life of attenuatedvaccines present problems, but this can be overcomein some cases by the use of viral stabilizers (eg, MgCl

2for poliovaccine).

Interference by coinfection with a naturally occurring,wild-type virus may inhibit replication of the vaccinevirus and decrease its effectiveness. This has been

noted with the vaccine strains of poliovirus, which canbe inhibited by concurrent infections by variousenteroviruses.

Documents

26 Virology